Where is the Displacement node in Blender?
The Displacement node is an upcoming feature - this is an example of the documentation being ahead of the official Blender release.

You’ll note that in the example image from the documentation the Material Output Displacement socket is a Vector type (it’s blue). At all ‘current’ releases of Blender the Displacement socket is a Scalar value (grey) and would act as displacement along the surface normal.

The Displacement node effectively allows you to generate the Vector displacement from a Scalar value to effectively mimic the displacement available at 2.79 of Blender. The new Vector displacement will be an exciting new feature (for me, at least) and should hopefully be a point where the experimental ‘true’ displacement becomes standard supported functionality (rather than ‘experimental’ and subject to change - as it has here!).